Online ftp backup dropping connection


login to the ftp backup server of the online plan is not stable I have a lot of dropping connection , any hints ?

jbrange% ftp -n
Connected to
220 server ready - login please
ftp> user xxxx yyyy
331 password required
230 login accepted
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
421 Service not available, remote server has closed connection.